The invention relates to a cathode ray tube for displaying coloured pictures comprising in an evacuated envelope means to generate a number of electron beams, a display screen comprising a large number of areas luminescing in different colours, and colour selection means comprising a large number of apertures, in which apertures a magnetic quadrupole lens is formed, and which apertures assign each electron beam to luminescent areas of one colour.
Such a cathode ray tube of the post-focusing type is disclosed in Netherlands Patent Application No. 7515039. The object of post-focusing is to increase the brightness of the displayed picture by increasing the transmission of the colour selection means. In tubes without post-focusing a very large part, for example 80 to 85%, of the electrons is intercepted by the so-called shadow mask. By using post-focusing, the apertures in the colour selection means can be enlarged since as a result of the focusing in the apertures the electron spots on the screen are considerably smaller than the apertures so that nevertheless sufficient space is present between the electron spots of the various electron beams.
In the known cathode ray tube of the post-focusing type a magnetic quadrupole lens is formed in each aperture of the colour selection means. In these quadrupole lenses the magnetic field is substantially perpendicular to the electron paths and the electron beams are focused in one direction and are defocused in a direction perpendicular thereto. According to an example of the known tube the colour selection means are formed by a grid of two sets of parallel strips of soft magnetic material which cross each other and which are interconnected at the crossings by means of slices of hard magnetic material. The slices are magnetized in a direction perpendicular to the grid. The positioning of the individual slices of hard magnetic material at the crossings of the grids of strips of soft magnetic material, however, is a cumbersome method for the manufacture of large numbers of tubes. According to another embodiment the colour selection means are formed by a ferromagnetic plate or a non-ferromagnetic plate having a layer of magnetizable material in which around each aperture a magnetic quadrupole is written by means of a writing head. In addition the colour selection means must be provided with extra means to screen the electron beams from the earth's magnetic field because a ferromagnetic plate which is permanently magnetized does not screen the earth's magnetic field.